Combustion chambers for burning natural gas or premixed fuels with air are used in such divergent applications as central heating, metallurgical processing furnaces and gas turbines. The different requirements for these various applications have resulted in the development of several types of burners for mixing the fuel with air, or another source of oxygen, igniting it, and burning the resulting mixture. Such burners include the familiar Bunsen burner, in which the flame is stabilized by the rim of the burner, the surface combustor, in which an incandescent surface is used to promote and maintain combustion, and aerodynamic combustors such as the toroidal burner and the cyclone burner. In the toroidal burner, a bluff body is introduced into the path of flow of premixed fuel and air, to produce a wake acting to stabilize the flame at relatively high rates of flow by increasing the average residence time of the combustible mixture in the region of the flame. In a cyclone burner, a vortex generator is installed in the flow path upstream of the flame to produce a vortex that also serves to increase the residence time of the gas at the flame.
Bunsen burners are unsuited for high intensity applications because throughflow velocity is very low, generally a few feet per second. Surface combustors are hard to start, since they need to have their walls preheated. They are also limited in life because no known material will last indefinitely if heated to incandescence. Metals tend to oxidize when so heated, and ceramic materials are subject to thermal shock. Accordingly, aerodynamic burners are preferred where high combustion intensity is desired. However, aerodynamic burners of either the toroidal or the cyclone type operate only over a rather narrow range of flow rates (about 4 to 1) and the combustion intensity produced by such burners is limited. Of these, the toroidal burner produces the higher intensity, but with the larger pressure drop. It is also susceptible to damage by flames hitting the walls, and is correspondingly difficult to design.
The primary objects of my invention are to increase the combustion intensity obtainable with aerodynamic gas burners; to increase the range of flow rates and fuel-to-air ratios over which such combustors can be operated; and to provide a selfcooled burner. Additional advantages include ease of starting, simplicity of construction, and the practicability of designing reliable burners in smaller sizes than conventional closed burners.
Briefly, the aerodynamic fuel combustor of my invention comprises a novel flameholder connected between a combustion chamber and a mixing chamber. Means are provided for admitting desired proportions of fuel gas and air to the mixing chamber to form a combustible gas under a controlled pressure. The flameholder comprises a vortex generator including at least one, and preferably two or more flow channels, shaped to supply a vortex of swirling gases from the mixing chamber to the combustion chamber. The flow channels are formed by airfoils terminating in bluff trailing edges of substantial area, causing eddying in the flow.
I have discovered that a surprisingly high combustion intensity can be achieved in a combustor so constructed, with a remarkably small accompanying pressure drop. In addition, a stable flame can be attained over a wide range of fuel-to-air mixtures and flow ranges. A further advantage arises from the fact that cooler portions of the burning gases spend more time in the outer region than in the inner region of the combustion space, helping to keep the walls of the combustion chamber cool.
As applied to gas turbines, the fuel combustor of my invention improves efficiency by greatly reducing the irreversible pressure loss through the fuel burning zone. Such a pressure loss subtracts directly from the available power. For stationary applications, the high combustion intensity attainable makes it possible to use smaller apparatus to effect the same heating purpose and the combustor itself may be made in smaller sizes than is practical with other aerodynamic combustors. The high exit gas velocity obtainable promotes heat transfer in industrial applications such as heat-treatment and melting furnaces. Since fuel and air flow rates may be varied over a wide range, proportional control of temperature, rather than on-off control, is possible. Since burning takes place in a conduit, as it does not in conventional aerodynamic burners, it is possible to ensure that all gases are burned before they mix with surrounding gases, thus improving combustion efficiency. Finally, the combustor of my invention may be designed for a particular application with ease, because burning out of the casing is eliminated as a problem, and because the passages are large and operate at low temperatures, preventing deterioration due to clogging or corrosion.
According to another embodiment of my invention, the improved combustor is combined with a jet ejector or jet pump for the purpose of obtaining the maximum combustion intensity in a burner of relatively small size. This is of particular advantage in selfcontained portable units which use pressurized gas tanks as the fuel source, such as are commonly used by plumbers, roofers and other artisans, as well as by home craftsmen. The jet ejector is joined for serial flow with the combustor in a common torch, which is conveniently mounted on a fuel tank or connected thereto by a flexible tube.
I have found that torches of the type described have a number of advantages over prior torches. These advantages include the fact that torches made according to my invention can burn a stoichiometric mixture of fuel and air without blowing out. Conventional torches use a fuel-air mixture which is rich in fuel; this reduces the velocity of the gas through the torch and thus prevents blowouts. However, to complete combustion, these prior torches used secondary combustion with ambient air at a location downstream of the burner. This secondary combustion is undesirable because it reduces flame temperature as the ambient air cools the flame. The result is that the heating effectiveness of such prior torches is substantially reduced.
Because of the improved burner construction used in torches incorporating my invention, the flame stability is sufficiently high that the torch will burn with a stoichiometric fuel-oxidizer mixture without blowout. Thus torches made according to my invention reach maximum flame temperatures and heating effectiveness. Typically torches of my invention supply hot gases at temperatures about 500.degree. F above conventional torches using the same fuel.
A second problem of all portable torches of the type described is that the torch must use the energy stored as fuel pressure in the fuel tank to propel the combustile gas. In prior torches, much of this stored energy was used for flameholding and relatively little was converted to gas velocity. In torches using the improved combustor of my invention in combination with a jet ejector, a much smaller fraction of the available energy is used for flameholding and a much larger fraction is used to impart velocity to the gas.
As a result of the features of the torch of my invention in use it supplies much higher velocity gases at much higher temperatures than conventional torches, resulting in much higher exit velocities and therefore higher heating effectiveness than prior torches using the same fuel and oxidizer.
The ejector pump used with the torch includes a nozzle for injecting fuel into a diffuser through an entrance chamber having suitable openings for the injection of combustion air. The diffuser delivers the fuel-air mixture into a torch tube having the mixing chamber, flameholder, and combustion chamber at its outlet end. The design of the ejector pump is in itself conventional. In combination with the ejector pump, the improved combustion is observed to produce flame temperature that are 25 per cent higher than prior torches, gas velocities 50 per cent greater than prior torches with resulting heating rates that are 75 per cent higher than presently available conventional torches.
The manner in which the fuel combustor of my invention is constructed, and its mode of operation, as well as the construction and mode of operation of a portable torch using the combustor will be made clear by the following detailed description, with reference to the accompanying drawings, of various embodiments thereof.